1. Field of the Invention
The present invention relates to an HC (hydrocarbon) emission control member for an exhaust gas, which is used in an exhaust system in an automobile or the like, and particularly, to an improvement in an HC emission control member for an exhaust gas comprising an HC adsorbing layer on a carrier and an HC oxidizing layer on the HC adsorbing layer.
2. Description of Prior Art
There are conventionally known exhaust emission control catalysts of various configurations used in automobile exhaust systems. However, a common exhaust emission control catalyst is able to purify an exhaust gas by oxidation only if the exhaust gas is at a high temperature, e.g., about 180.degree. C. or more and hence, the catalyst has an extremely low ability to purify an exhaust gas, which includes hydrocarbon (HC) at a high concentration, having a low temperature immediately after the start of an engine.
An HC emission control member, which is conventionally known, comprises an HC adsorbing layer made of an aluminosilicate (zeolite) to catch HC in an exhaust gas having a low temperature (for example, see Japanese Patent Application Laid-open No. 2-56247).
However, the aluminosilicate adsorbs water along with HC in the exhaust gas and the water adsorption reduces the HC adsorbing ability of the aluminosilicate. When the aluminosilicate desorbs HC, the adsorbed water is also desorbed. For this reason, it is difficult to raise the temperature of the HC oxidizing layer to a temperature enough to activate the HC oxidizing layer due to the heat of vaporization produced by water evaporation. As a result, the conventional HC emission control member could not satisfactorily adsorb and desorb HC.